The present invention relates to a process of using steam to assist achieving impregnation of lignocellulosic materials with various additives. The processing conditions are such that, in addition to impregnation, this process simultaneously achieves drying of the lignocellulosic materials.
The properties of lignocellulosic materials, such as paper, linerboard, corrugated and cardboard, can be improved to varying degrees by incorporating various additives to the materials. For example, it has been shown that the strength of linerboard in compression or tension can be substantially increased by incorporating sodium silicate or starch inside it. Usually the additives are in the form of a solution or a dispersion, and for the purposes of this disclosure, solution and dispersion may be used interchangeably. Similarly, for the purpose of this disclosure the terms agent, active, additive and saturant are used interchangeably. Finally, the terms incorporation, treatment, impregnation and saturation are used interchangeably for the purposes of this disclosure.
In general, this incorporation can be achieved using various methods, such as (but not limited to): 1) immersion of the lignocellulosic materials into a bath solution or dispersion of the additives, 2) spraying or brushing a solution or dispersion of the additives onto the lignocellulosic materials, and 3) coating (e.g. roll, blade, gravure, etc.) of the lignocellulosic materials with a solution or dispersion of the additives. However, the above methods do not achieve sufficient incorporation of the additives inside the lignocellulosic materials. In most cases, this results in minimal property improvements.
One treatment method that solves the problem of insufficient incorporation of additives is described in U.S. Pat. No. 5,776,546, issued to Long, and assigned to MiPly Equipment Inc. The MiPly process uses one or two converging pressure chambers (e.g. in the form of a journal bearing) to achieve paper web impregnation with various additives. However, when the solvent (or its major part) of the additive solution or dispersion is water then there is typically a need for drying after the MiPly process. However, U.S. Pat. No. 5,776,546 does not disclose nor teach the simultaneous drying of lignocellulosic materials. This drying can be achieved in various processing equipment in series with the MiPly process, such as cylinder dryers, air flotation dryers, impulse dryer, Condebelt dryer, superheated steam dryer, etc.
The Condebelt drying process is described in U.S. Pat. No. 5,722,182, invented by Lehtinen, and assigned to Valmet Inc. In the Condebelt process the paper web is carried on a band formed of two permeable wires (in the form of a fine and a coarse screen) and fed between two smooth steel bands. The upper band is kept hot by contact with saturated steam and is used to apply pressure in the z direction (i.e., press drying) of the paper web. Typical pressure values are between 2 bars and 5 bars (between 29 psi and 72.5 psi), while the maximum pressure is 10 bars (145 psi). The temperature values of the upper band are between 130xc2x0 C. and 160xc2x0 C. (between 266xc2x0 F. and 320xc2x0 F.), while the maximum temperature is 180xc2x0 C. (356xc2x0 F.). The lower band is water-cooled and kept at lower temperature, typically less than 90xc2x0 C. (194xc2x0 F.). According to Valmet""s publications, the z-directional pressure and the accompanied elevated temperature of the upper band have been found to: 1) plasticize the fibers, 2) cause flattening of the fiber-to-fiber bonds, 3) cause softening of the fiber surface material (i.e., lignin and hemicelluloses) and flowing to form crescent-shaped comer weld bridges between two fibers, and 4) increase the paper web density. All the above effects result in improvements in the dry and wet strength properties as well as other properties. A typical increase in the strength of linerboard dried with the Condebelt process has been reported to be up to 30%. However, U.S. Pat. No. 5,772,182 does not disclose or teach the adding and impregnating of additives into lignocellulosic materials during the drying process.
Another drying process uses superheated steam supplied from an external source to evaporate the water inside the paper web. U.S. Pat. No. 5,210,958 issued to Bond et al., and assigned to McGill University and the Pulp and Paper Research Institute of Canada describes the use of impinging superheated steam (i.e., exogenous steam) to dry paper webs. However, U.S. Pat. No. 5,210,958 does not disclose or teach the adding and impregnating of additives into lignocellulosic materials during the drying process.
What has been missing is a process that uses steam to assist in achieving impregnation of lignocellulosic materials with various additives while providing simultaneous drying of the lignocellulosic materials.
The present invention relates to a process that uses steam to assist achieving impregnation of lignocellulosic materials with various additives. This steam-assisted process can achieve simultaneous drying of the lignocellulosic materials. Therefore, the properties of the lignocellulosic materials are improved and the number of serial processing steps is reduced.